The product for sale at 29 yuan per catty is not gānmā 乾媽 ("godmother" in Mandarin) but rather kam-á 柑仔 ("tangerine" in Taiwanese). They have borrowed the Mandarin near-homophone gānmā 乾媽 ("godmother") to write the Taiwanese kam-á 柑仔 ("tangerine").

Heaven forbid that one should say or think gànmā 幹媽 ("'do' [your] mother") when reading gānmā 乾媽 ("godmother" < "dry + mother") — they both would be written as 干妈 in simplified characters.

And none of this has anything to do with "gànma 幹嘛 / 干嘛?" ("why?") or gànmá 幹嗎 / 干吗 ("why on earth? whatever for? what's going on? what's wrong? what are you doing? what's up? what to do?"). N.B.: some speakers insist on keeping these two interrogatives separate, but others collapse them into one.

Greg said,

@VM What's your thought on the impetus for this? 柑仔 is fewer strokes than 乾媽, so the substitution seems odd on a written sign. Is it cutesy? Fashionable? Representative of some shift in the relationship between Mandarin and Hokkien in Taiwan?

My immediate response is that kam-á 柑仔 ("tangerine") is overtly marked as Taiwanese, which the vendor might have felt would be off-putting to Mandarin speakers. Add to that the cuteness of the pun, and writing it as gānmā 乾媽 ("godmother") must have been nearly irresistible.